I don't see how ADAP6 will mail to you in the first week of July when both amazon.ca and amazon.com say the book won't arrive until the 13th. In my time zone, the 13th falls into the the second week of July - actually the last day of that week (Sat). I'm expecting my copy to arrive in the fourth week of July.

Really, if your time warp machine works very well you could get ADAP6 before Doug wrote it and tell him about all the mistakes he's about to make. That would change the time-line in this universe, but it would just shift us into a different quantum universe as all possibilities and outcomes exist and happen... according to those with strings...

The brass section keeps its own time.

Have fun
Kevin O'Connor

Douglas did say it would be published on the first of July.

Actually, amazon.com states that the publication date is the 24th of june.

5mA to 10mA. It's a trade between the stage gain (which is, before any VAS degeneration, Au=gm*(Ri||R0)=40*Ic*(Ri||R0) where Ri is the output stage impedance and yes, a triple may allow a lower VAS current, for the same gain) and the VAS output impedance R0, which appears in parallel with Ri and is approximatively VA/Ic where VA is the Early voltage. So a large Ic may increase the transconductance, but decreases the stage load. If the output stage input impedance would be very large, then the VAS gain would be independent of Ic, unfortunately this approximation never holds.

Cascoding the VAS stage with a high VA device largely removes this limitation, then the maximum VAS current is limited mostly by the power dissipation and the SOA of the active device, and some stability criteria (some cascodes may be subject to potential local stability issues).

Quote:

Originally Posted by nigel pearson

One thing I would like to ask is what is an ideal VAS ( or trans-impedance stage ) current ? A stupid question I know . Still I will make a stab at it . Often I say 8 mA . 5.6 mA seems to often choose itself in my designs . On the face of it the more the better . My ears say not true . Perhaps it is simply like a jigsaw puzzle ? It all falls into place when right . Fixing one parameter helps that is certain . One thing that is worth asking is if using output triples does one need less ? If so do typical amps still run about 5 to 8 mA because it is easy for them to do so ?

Sorry if my question is too mundane for the advanced levels . Like the optimum cylinder size of an engine it is the sort of thing worth knowing . 330 cc set by Citroen in the 1920's I think ?

I am laying bets if answered at all it will be some need this and others need that . I think that's where I am at already . I am hoping for a pearl of wisdom here . My guess is 0.5 to 1 watt dissipation sets the VAS current?

I think the VAS transistor will easily sound different . It's inherent capacitance is all there is to know . I think in the Handbook it says internal capacitance not as nice as adding it externally . Naturally all people here know that . However it is overlooked elsewhere . The classic example is people saying no VAS cap required if single input transistor . This is simply not true . It was the high capacitance transistors of old that made that possible . Gain is an obvious problem so I won't state it . My guess is it is like the high jump , clear the pole and the sound should be the same ?

I have previously explained to you that my circuits are to be the subject of a future publication and will, therefore, not be posted here.

You mean .. you mean you only have ONE good circuit and all your others are cr*p?

If this is the case, I'm sure Jan Didden will make an exception and allow us a sneak preview of your masterpiece. It will whet our appetite to buy the next issue of Linear Audio and may even persuade indigent beach bums to get a job.

Quote:

Further more your assertion that I "muck up someone else's design" is inutterable nonsense.

Err.rh! When you modify someone's circuit, it seems to be invariably worse. I haven't seen ALL your 'improvements' but for those you recommended for my circuits, THD is sadly at least 20dB worse and stability too. Is there some esoteric improvement I've missed that compensates for these 'trivial' points?

Quote:

I have, for instance, demonstrated that your obsession with Cherry's output stage inclusive Miller compensation is entirely misguided as the scheme is unstable, and I have, moreover, given sound reasons why this is so. I showed that for the scheme to work you would need significant shunt compensation to ground at the TIS output which would merely compromise second stage linearity and the slew rate of the amplifier.

Err.rrh! I've posted at least 3 of my own circuits where this isn't the case in SPICE world and more on astx & dadod's circuits .. and more importantly, mine are stable in the 'real world'. None of them require evil shunt caps at the Holy HiZ VAS output.

You asked explicitly for examples and I provided you with a long list on at least 2 occasions.

I totally accept that YOU have the ability to take one of my circuits and make it unstable in SPICE world and probably in 'real life' too. You are the MASTER at this. But then, you seem to put little credence to 'real life' compared to your sims. and (perhaps wonky?) theories.

You seem to be a master at another useful technique : When you ask for evidence and it is presented in detail, you conveniently forget it happened.

I now count at least 4 people who have posted SPICE simulations of this useful property of VAS degeneration. Perhaps we are all cerebrally challenged and only you know how to use SPICE properly. You certainly seem to be the one that is most reliant on SPICE to the exclusion of 'real world' considerations.

Quote:

Have you actually tried to replicate or otherwise perform my simulations with the "correct" circuits and models of your choice? Hell No!

I promised you and Bob the results but I didn't post them cos the example (like all of yours I've seen) had poor choices of currents & operating points such that stability & THD were marginal. Mea maxima culpa

This is the Inner Loop.

You'll note without R10, the VAS emitter resistor, the stability is seriously wonky. Is this the case with ALL your designs? Adding the emitter resistor and my lucky decoupling cap C3, improves stability.

Should point out that the context was suggesting to Bob that decoupling the VAS emitter resistor was another useful string to the stability bow. Some of us are always on the lookout for new & better dodges to improve stuff ... rather than degrade stuff. BTW, the effect of VAS emitter resistor & judicious decoupling is apparently a 'Bode Step' to da pedants. I prefer to think of it as a kink to Nyquist.

I didn't do more work on this as your design (Unobtainium devices, no Zobels or output Inductor etc) would at best, be marginally stable with real loads and have poor distortion. It's performance is easily beaten by a much simpler, more stable, 'real life' circuit using my favourite 'pure Cherry' or even lesser compensation methods.

Quote:

Indeed, all you've done in response is captiously cling to extraneous and wholly impertinent minutiae such as whether the quiescent TIS current differs by a few milliamps and the like, seemingly entirely oblivious to the fact that an amplifier's major loop and minor loop stability should be completely insensitive to TIS quiescent current or the degeneration resistors used in the current mirrors and other irrelevant trivia.

Not so. If eg Iq is inadequate, there are often stability effects. This is sometimes seen as bursts of oscillation on the THD residual or just higher THD on very specific loads. You may like to investigate this in the 'real world'.
________________

If you don't mind, I won't waste time looking at any more of your circuits ... unless you tell us that they illustrate good performance & practice. Looking at an endless line of cr*p circuits is usually NOT productive.

Of course if you post a good circuit with good performance, we will be eager to learn how you do it ... especially if this is backed up with 'real life'

BTW, the effect of VAS emitter resistor & judicious decoupling is apparently a 'Bode Step' to da pedants. I prefer to think of it as a kink to Nyquist.

Well, a Bode Step is a rather specific response function, not just any arbitrary kink.
Nyquist is just a particular presentation of the response.
Experts like Lurie and Horowitz find Nichols plots more convenient for this kind of work and I have started to think so too.
You may want to try them.

I just ran your sim. and found that Michael does have a point.
If R10 is simply increased from 0.1 to 22 ohms then the stability does deteriorate.
Of course the increased value of R10 does allow freedom to increase C3.
But it does appear that the simple increase in R10 does destabilize that circuit with those models.
I wish Michael's models were better, now I have to decide whether to spend the time to determine if it is someone's error, a quirk, or more fundamental.

I just ran your sim. and found that Michael does have a point.
If R10 is simply increased from 0.1 to 22 ohms then the stability does deteriorate.
Of course the increased value of R10 does allow freedom to increase C3.
But it does appear that the simple increase in R10 does destabilize that circuit with those models.
I wish Michael's models were better, now I have to decide whether to spend the time to determine if it is someone's error, a quirk, or more fundamental.

Damn. Caught out by the Head Prefect. Dis was Michael's attempt to dis one of Bob Cordell's circuits.

Yes. I knew but was hoping no one would notice I didn't keep my other sims of Michael's stuff cos they all gave awful THD & stability.

As I said, the original intention was to suggest yet another stability dodge to Bob.

I think all Michael's models need A LOT of work to come up with anything worth keeping. Try a 8R load and doing THD20k with this one ... or indeed any of his others.